A solid oxide fuel cell (SOFC) has been known as one of devices for generating electric power by using fuel cells. The solid oxide fuel cell is superior over a proton exchange membrane fuel cell (PEMFC) in long-term stability and reliability and besides, has higher energy conversion efficiency.
However, a solid electrolyte used as a solid oxide electrolyte exhibits low ion conductivity at low temperatures and hence, need be raised to such a high temperature in the range of 1000° C. to 800° C. in order to provide for an effective power generation. Further, the conventional solid oxide fuel cell suffers from a drawback that heat loss makes it difficult for the fuel cell to perform a thermally self-sustained operation as maintaining high power generation efficiency.
In this connection, apparatuses equipped with such a solid oxide fuel cell and designed to perform the thermally self-sustained operation have been proposed in Japanese Unexamined Patent Publications No. 2004-71312 and No. 2004-71315. The thermally self-sustained operation means an operation to maintain the fuel cell at temperature above a power generation temperature for effective power generation without applying heat from an external source but relying only on heat produced by the fuel cell itself. The smaller the apparatus, the greater is the rate of heat loss. Therefore, the smaller apparatus has the greater difficulty in performing the thermally self-sustained operation and the greater tendency to be decreased in the power generation efficiency.